if you are going through some expensive academy to "learn" dog training and quote OP " I plan on being a professional pet dog trainer, but realistically I will be working as a dog walker to earn the bulk of my income the first few yea after dog training academy." , you should have some skills in managing a dog.
Think of all the variety in temperaments , energy , and training-or-not that you will have in your clients dogs, especially if you are going to walk or transport or dog-park with several dogs at one time.
Are you sure you are comfortable with the GSD as a breed?
OP "I want as clear a picture as I can get about living with a working line GSD."
Once again there can be a variety of drives and emphasis , energy etc, just as there can be with the show lines .
If you want to do SAR the selection, raising and training and environment the dog is in are very important .
The focus has to be on the development of that one on one relationship , the finesse of reading the dog , the development and building of drives, and rewards structure. So much involvement with other dogs is going to erode some of the focus , constantly rewarded with dog-party atmosphere .
If you want to do SAR then you have to find the real deal not something that looks the part , and that includes drive and endurance and tenacity , a fire in the belly for the work
Any ways to ensure (or increase chance of) dog's approval in SAR?
I think you still need to think some things out before you go looking for a breeder.